I just noticed that my oscar appears to be breathing only through one gill? Is this normal? Or can it be from a parasite or disease?

Welcome to the site

It could be something or nothing. See if you can compare the gill that seems okay to the one that isn't. The healthy gill should be a nice healthy red colour. If the other gill is different then there may be problems, what problems? I couldn't tell you at the moment.

And as with any post regarding to health, but please ask you to test the water for ammonia, nitrite, nitrate and pH. Without knowing the quality of your water, we are to a certain extent shooting in the dark.

oscar can do the trick by vibrating one gills only.
but it doesn't mean he/she had problem with the other
side of the gills. just make sure your water is in good
condition, no need to worry.

Yes thank you all for your feedback. He is breathing through both gills. It appeared he wasn't at one time but I noticed last night he was doing OK. I have recently cleaned out my whole tank as it appears my buddy has HITH. I am so upset as I feel it is my fault for not maintaining my tank well enough. He is eating and very active so I am hoping I caught it in time. How long does it take to get rid of the spots on his head?